The electrical activity in the brain produces detectable signals known as brain waves, the frequency of which is associated with changes in mental state and cognitive ability. During daily waking life, a normal human brain functions at frequencies ranging from 13 to 25 Hertz (Hz), or cycles per second, called the beta state. During times of rest or daydreaming, brain wave cycles drop to 8 to 12 cycles per second, or the Alpha state. When sleeping, Theta and Delta states are dominant, ranging from 4 to 7 and 0.2 to 4 cycles per second respectively.
Brain waves can be altered if a person is presented with precisely timed, repetitive stimuli through the ears or eyes at repetition rates equal to the desired brain wave frequency. This is called brain wave entrainment. For example, if a 5 beat per second audio loop is played to a subject over a period of 6 minutes or more, the subject's brain will entrain to 5 cycles per second, or brain activity at the 5 Hz frequency will increase. FIG. 1A depicts a typical brain wave entraining beat with a pure tone. FIG. 1B shows a brain wave response to the beat. Because brain waves relate to mood and attention levels, researchers believe that stimulating frequencies can assist in a variety of activities, including relaxation, concentration, sleep and more.
Previous inventions have used flashing lights or pulsing tones to produce brain wave entrainment, as shown by U.S. Pat. Nos. 4,315,502 and 5,289,438.
However, pure tones and tone pulses can be quite invasive, grating and distracting to many people. Previous inventions have endeavored to solve this problem by masking over the entrainment with white noise, such as in U.S. Pat. No. 5,213,562, or music. Unfortunately, masking the brain wave entraining sounds of an audio composition can decrease the effectiveness of the brain stimulation. Additionally, while the masking acts to hide the harsh tones used, it does not eliminate them. Often the tones can still be heard distantly and can interfere with any added music or sounds.
Music alone has been used to affect the brain such as in U.S. Pat. No. 5,586,967, which describes a method and apparatus for inducing enhanced states of learning by presenting musical sounds in ascending and descending crescendos. Unfortunately the tempo of music stimulating brain waves is limited since brain wave frequencies range as high as 60 cycles per second.
Other methods have endeavored to tackle this problem by applying modulations non-specifically to entire pieces of music. Modulations alter sound to create a pattern of beats where none existed before. FIG. 2A depicts a sound file prior to modulation and FIG. 2B shows the same sound modulated to produce a brain wave entraining beat, using amplitude modulation. But the basic problem remains, since the only way to create subtle, non-invasive audio entrainment in this case is to decrease the intensity of the modulation, thus decreasing the overall effectiveness of the brain wave entrainment.